CbpM and CbpG of Streptococcus Pneumoniae Elicit a High Protection in Mice Challenged with a Serotype 19F Pneumococcus

Among the many pneumococcal antigens, choline-binding proteins (CPBs) display high immunogenicity in animal models. This study aims to determine the immunogenicity of CbpM, CbpG and CbpL proteins in a mice model. The genes were cloned into pET21a expression vector and the recombinant proteins were produced. Mice and rabbits were immunized with the purified recombinant proteins. Subsequently, the mice were challenged with S. pneumoniae and their survival as well as bacterial clearances were followed. The antibody responses of the mice immunized with recombinant proteins were determined by ELISA assay. The opsonophagocytosis assay was performed using rabbit’s sera. Passive immunization was also carried out in another group of mice using two doses of anti-CbPs antibodies. Finally, these mice were challenged and their survival as well as bacterial clearance were determined. The mice actively immunized with CbpM, CbpG and CbpL recombinant proteins showed survival rate of 100%, 85% and 75%, respectively. The survival rates among passively immunized mice groups which received100µg/ml dose of anti-CbpM, anti-CbpG and anti- CbpL were 50%, 50% and 25%, respectively. For all groups, however, only 25% of mice which received 10µg/ml dose of antibody survived after bacterial challenge. The rates of opsonization with anti-CbpM, anti-CbpG, and anti-CbpL antibodies at 100 and 10µg/ml doses, were found to be 45.6 and 14.7%, 82.3 and 12.9% and 12.2 and 9.35%, respectively. Our findings suggest that the recombinant proteins particularly CbpM and CbpG can protect the mice against pneumococcus19F serotype and effectively induce a protective antibody response. Thus, CbpG and CbpM proteins might be used as suitable vaccine candidate in pneumococcal vaccine formulations.